High-frequency oscillations (HFO) including Ripples are associated with well-defined neuronal events and are commonly recorded in certain parts of the mesial temporal lobe (MTL) such as the CA1 region and the subiculum. Conspicuously, such oscillations cannot be observed under normal conditions in the dentate gyrus, but this structure will readily produce HFO during the process of epileptogenesis. Because of their tight association with MTL epilepsy (MTLE), HFO of the dentate gyrus should always be considered to be pathological. Moreover, the MTLE-associated HFO observed with high incidence in the CA1 and subiculum may also significantly differ from the HFO seen under normal conditions. The MTLE-associated pathological HFO (pHFO) consisting of pathological Ripples (pR) in the range of 100-200 Hz and Fast Ripples (FR) in the range of 200-500 Hz in epileptogenic structures reflect fundamental neuronal mechanisms responsible for the development of epilepsy and the generation of spontaneous seizures. Since inhibitory interneurons play a prominent role in the manifestation of these synchronous events, the immediate research objectives of this proposal consist of a series of tightly integrated parallel in vitro studies in slices from MTLE patients and from an experimental animal model of this condition: the pilocarpine-treated (PILO) mouse. The goals are i) to identify and characterize the alterations in principal neurons and in specific subclasses of interneurons leading to re-assembly into pathological networks capable of generating pHFO;ii) to examine the effects of ablation and stimulation of specific interneurons on pHFO generation. The overall hypothesis is that secondary to changes in intrinsic neuronal excitability and interconnections during the process of epileptogenesis, patterns of activation and spiking, that differ from control conditions, in specific interneurons and principal cells contribute to the generation of pHFO. In some MTL structures (e.g., CA1 and subiculum), the cell assemblies involved in pHFO generation may not be the same as those active during physiological Ripple activity. In the dentate gyrus, where normally there are no HFO generating cell assemblies, a pathological reorganization of specific interneurons and dentate gyrus granule cells (DGGC) into epileptic cell assemblies capable of generating pHFO is a critical component of epileptogenesis and may constitute.one of the earliest events of this pathological process. Thus, identifying the progression, targets, and mechanisms of such pathological cell assemblies will yield important clues for preventing and possibly even reversing the epileptogenic process.